Brake failures, conditions and capacity have been measured before by passive temperature sensors, passive ultrasound sensors, etc., as shown for example in U.S. Pat. No 4,591,213. See also for example column 8, lines 6-19 of U.S. Pat. No. 4,743,074 to Inoue which shows a control system which includes the use of passive ultrasonic sensors. Its main function is to prevent audible brake squeal. Vibrations and brake force are also measured. Details on ultrasound sensors may be found in Siemens publication "Ultrasonic Remote Sensors for Noncontact Object Detection" by Kleinschmidt and Magori, reprinted from "Siemens Forsch.-u. Entwickl.-Ber." Vol. 10 (1981) No. 2, pp. 110-118.
However, these systems are relatively complex, and generally are not particularly reliable. In a passive ultrasound system, a fault condition is detected by sensing vibrations within the system; however, if the sensor fails, the fault will go undetected.
In elevator systems, as another example, switches have been used to determine the status of the brake. When the elevator car is landing at a floor, the drive system is still active. It would be desirable to determine at this point if the brake will be capable of working, i.e., ahead of time. This must be done with no load on the brake because the car may be at or near balanced load. This cannot be done with a friction force measuring sensor or any other of the current methods. A switch won't give an indication if the brake has dropped but only if one of the linkages has moved enough to cause activation of the mechanical switch. This arrangement will not detect several failure conditions, such as worn pad, misaligned shoes/pads, missing shoes/pad or linkage failures. The switch doesn't measure force, but only displacement of the linkage.